In C3H/HeJ mice infected as neonates with Moloney murine leukemia virus (MuLV-M-carriers), autoaggressor lymphoid cells appear in the thymus and the peripheral lymphoid tissues during the course of lymphoma development. These cells are vigorously reactive against normal syngeneic and some allogeneic target cells; they are not reactive against xenogeneic target cells and they also spare syngeneic target cells infected with leukemia viruses. The aggressor cells have been characterized as MuLV-infected, thymus-derived (T) cells. Lymphoma cells arising in MuLV-M-carriers demonstrate identical reactivity. The purpose of the proposed studies is to gain further insight into the phenomenon of MuLV-M-induced autoaggression, its relationship to lymphomagenesis, and its applicability as a model for virus-induced autoimmunity. Employing conventional methods for studying cell-mediated immune responses, we shall attempt to: 1) Further characterize the lymphoma cells arising in MuLV-M-carriers with respect to their immunological markers. 2) Determine whether autoagrressor thymocytes appear before malignant lymphoma cells. 3) Determine whether autoaggressor thymocytes can be triggered after MuLV-infection in vitro. 4) Determine whether an immunoregulatory hormone such as thymosin or a virus such as the lymphocytic choriomeningitis virus can interfere with the development of autoaggressor thymocytes in MuLV-M-carriers. 5) Further understand the relationship between autoaggressor thymocytes and immune suppressor thymocytes in MuLV-M-carrier mice. 6) Determine whether the autoaggressor activity seen in MuLV-M-carriers also occurs in association with lymphomas induced by endogeneous MuLV.